1. Field of the Invention
The present invention relates to an apparatus for carrying out a Standard Penetration Test (SPT) to determine the penetration resistance, geological distribution and nature of the soil, and more particularly to an automatic hammer system for a standard penetration test, which enables its hammer to fall from a precise predetermined height regardless of a penetration depth of a sampler, and is able to automatically carry out sequential test procedures such as counting the number of blows by the hammer and a penetration depth of a sampler according to the number of blows.
2. Description of the Prior Art
To undertake various civil engineering works and construction works, there is a need to first determine the penetration resistance, geological structure and geological composition of the soil by checking consistency and relative density of the soil by testing the soil of an area in question. To this end, a test procedure known as the xe2x80x9cStandard Penetration Testxe2x80x9d is commonly used.
The standard penetration test is a representative geological surveying test for estimating soil constants such as strength, relative density and angle of internal friction of ground in question, which is carried out as follows. A hammer of 63.5 kg is raised to a height of 75 cm and then released to fall and impact a split barrel sampler (referred to merely as a sampler, hereinafter), and this procedure is repeatedly carried out until the soil is penetrated to a depth of 30 cm by the sampler. Subsequently, an N value, which is the number of blows of the hammer counted until the sampler penetrates the soil to the depth of 30 cm, is calculated, and the soil constants of the ground are obtained from the N value.
In this test, the number of blows counted until the sampler initially penetrates the soil to a depth of 15 cm is regarded as a number of preliminary blows because the soil sample is believed to be disturbed, and the number of blows counted until the sampler further penetrates the soil to a depth of 30 cm from the level corresponding to the initial depth of 15 cm is determined as the N value for the soil in question. Where the number of blows counted until the sampler penetrates the soil to the depth of 30 cm exceeds 50, a depth of the soil penetrated after the hammer gives the sampler 50 blows is measured.
As a rule, though the standard penetration test must be carried out every 1.5 m under the current ground surface, the standard penetration test is carried out only once where the same geological formation continues underground.
Referring to FIG. 1, there is shown the most common apparatus for use in the standard penetration test, which uses a winch.
As shown in the drawing, a frame 1 is provided at its lower portion with a winding drum 2 fixed thereto, and is provided at its upper portion with a pulley 3. A rope 4 is wound around the winding drum 2 for several turns and wrapped around the pulley 3 to be directed downwardly. A cylindrical hammer 5 is coupled to one end of the rope 4, and slidably inserted over a vertical guide rod 6.
The guide rod 6 is coupled at its lower end to a drill rod 8, which is inserted into a boring hole (not shown) which has been previously drilled. The drill rod 8 is provided at its upper end with an anvil 7 mounted thereon, on which the hammer 5 impacts, and is provided at its lower end with a sampler (not shown) coupled thereto to obtain a disturbed soil sample. The guide rod 6 is provided with a marking which indicates a maximum lifting height at a certain height from the anvil 7.
In an operation of the winch-type apparatus, the drill rod 8, on which the sampler is mounted, is inserted into the boring hole of the soil, and then coupled to the guide rod 6. Subsequently, the rope 4 is pulled by an operator to raise the hammer 5 to the lifting height (75 cm), and then released to allow the hammer 5 to free fall. Consequently, the hammer 5 falls along the guide rod 6 and impacts the anvil 7.
Therefore, the impact of the falling hammer 5 is transmitted to the drill rod 8 through the anvil 7, so that the soil in question is penetrated by the sampler coupled to the lower end of the drill rod 8. This procedure is repeated until the penetrated depth reaches a desired value.
However, since such a conventional winch-type apparatus for use in the standard penetration test is required for an operator to check, with his naked eye, a lifting height of the hammer 5 during every lifting procedure, it is difficult to maintain a constant lifting height throughout all the striking procedures even though the test is carried out by a skilled person. Hence, the drill rod is applied with different impact strengths throughout the striking procedures.
Furthermore, since the hammer 5 is raised by the rope 4, frictional loss is generated between the winding drum 2 and the pulley 3 during the falling of the hammer 5. The frictional loss varies depending on the properties and age of the rope 4, and actual impact strength applied to the anvil 7 is reduced to a value lower than the specified value.
Therefore, the conventional winch-type apparatus is inadequate to carry out the standard penetration test, and it is difficult to assure a precise measurement of an N value and to assure reliability of test results because of various factors.
In addition, since an N value obtained by the test is in an operator""s memory, and a penetration depth of the sampler is obtained by an additional measuring procedure, an operator is apt to obtain incorrect test results, and considerably different test results may be obtained depending on operators even though the tests are carried out on the same soil sample.
To overcome the above-mentioned problems, a drive hammer system for a standard penetration test is disclosed in U.S. Pat. No. 4,405,020, which is adapted to enable a hammer to consistently fall from the same height, and to minimize frictional loss generated during the falling of the hammer.
The drive hammer system is slidably supported to an outer surface of a hydraulic cylinder via a pivot arm connected to a piston rod of the hydraulic cylinder. The hydraulic cylinder is vertically mounted on a drill rig. The pivot arm is rotated to a working position and raised by the hydraulic cylinder to be positioned over an impact surface of an anvil. When the drive hammer system is positioned over the anvil, a shutoff valve is opened to allow fluid in the hydraulic cylinder to be exhausted.
In this state, by actuation of a motor mounted on the cylindrical housing, a sprocket is rotated to cause a chain to be rotated clockwise. Lifting lugs on the chain are raised along a slot axially formed at the cylindrical housing by the rotation of the sprocket. At this point, the lug comes into contact with a lower end of a hammer received in the housing. As the lug pushes the hammer up, the hammer is gradually distanced from the anvil.
When the lug reaches the sprocket and begins to move outwardly, the lug moves from under the hammer, permitting the hammer to free fall until it strikes the impact surface of the anvil. By the striking action of the hammer against the anvil, a sampler penetrates the soil, thereby allowing the anvil to be lowered. At this point, the cylindrical housing free falls by the penetration depth of the sampler, and thus is placed on a flange of a drill rod, thereby maintaining a drop height at a certain value.
The drive hammer system itself is lowered by the penetration depth after every blow so as to maintain the drop height of the hammer at a certain value. However, since the drive hammer system strikes the flange of the drill rod soon after blows from the hammer (i.e. secondary blows), the sampler further penetrates the soil.
In addition, since the hammer is adapted to be raised by the lifting lug of the turning chain and to fall by release from the lug, the hammer may be raised to a position higher than the specified height by being struck by the lug in the course of turning when the chain is rotated at high speed.
In addition to this, it is troublesome to measure a penetration depth of the sampler by blows of the hammer by an additional measuring device.
Accordingly, this drive hammer system is not able to assure accuracy and reliability of an N value.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an automatic hammer system for use in a standard penetration test, which is adapted to enable a hammer to be raised and to fall automatically, and which is adapted to maintain a drop height of a hammer at a certain value, regardless of a penetration depth of a sampler.
Another object of the present invention is to provide an automatic hammer system for use in a standard penetration test, which is able to minimize loss of impact energy of a hammer caused by frictional contacts between associated components, and which is adapted to reliably prevent secondary blows against an anvil, thereby permitting the anvil to always be applied with a specified impact energy.
A further object of the present invention is to provide an automatic hammer system for use in a standard penetration test, which is adapted to automatically carry out a series of test procedures for counting the number of blows by a hammer and a penetration depth of a sampler according to the number of blows, thereby affording a precise N value.
In order to accomplish the above object, the present invention provides an automatic hammer system for a standard penetration test, comprising: a first vertical hydraulic cylinder rotatably coupled to boring equipment; a cylindrical housing positioned to be parallel to the first hydraulic cylinder and coupled thereto, the cylindrical housing being connected to a piston rod of the first hydraulic cylinder and adapted to receive therein an anvil of a drill rod, wherein the drill rod is provided at its lower end with a sampler to be inserted in a boring hole of the soil; a cylindrical hammer with a blind lower end, which is movably received in the housing to be disposed over the anvil; a holding assembly received in the hammer and adapted to hold the hammer at its lower dead point and to release the hammer at its upper dead point to allow the hammer to fall; a second hydraulic cylinder concentrically coupled to an upper end of the housing and adapted to raise and lower the holding assembly; means for limiting a lifting height of the hammer, which is received in the housing to be disposed over the hammer and integrally coupled to the holding assembly with a spacing therebetween, the limiting means being raised and lowered within a certain range; means for counting the number of blows of the hammer against the anvil; means for measuring a penetration depth of the sampler by blows of the hammer; and a control unit for carrying out control of the striking action of the hammer and calculation of an N value according to data obtained by the counting means and the measuring means, and for carrying out record and display of test results.
According to an aspect of the present invention, the holding assembly includes a cylindrical casing which is radially provided at its wall with a plurality of fitting slots at a certain angular spacing, a plurality of holding blocks slidably fitted in the fitting slots of the casing and adapted to selectively press an inner surface of the hammer, and a pusher unit received in the casing and connected to the piston rod of the second hydraulic cylinder, the pusher unit being adapted to outwardly push or release the holding blocks in the course of axial movement.
The pusher unit is adapted to outwardly push and release the holding blocks when the pusher unit is further lowered and raised after the limiting means is stopped.
According to another aspect of the present invention, the counting means comprises a detection slot formed at an upper portion of the housing, and a first sensor mounted on the plunger to detect the detection slot to count the number of blows by the hammer.
According to a further aspect of the present invention, the measuring means comprises a plurality of protrusions axially formed along an outer surface of the hammer at a certain pitch, and a second sensor mounted on a wall of the housing to detect the number of protrusions passed over the second sensor during every lifting motion, thereby enabling a penetration depth to be obtained from the number of protrusions.
According to the present invention, the holding assembly is actuated to outwardly press an inner surface of the elongated cylindrical hammer, thereby firmly holding the hammer. The holding assembly engaging the hammer is raised by the second hydraulic cylinder and then releases the hammer to fall freely. After a blow by the hammer, since the holding assembly holds the hammer at a position which is higher than the previous holding position by a penetration depth of the previous blow, a drop height of the hammer is uniformly maintained for every blow, regardless of a penetration depth of the hammer.
Furthermore, since the hammer is adapted to be raised to a certain height and then to fall therefrom without lowering displacement of the hammer system itself, it is possible to reliably prevent secondary blows caused by lowering of a conventional hammer system. Therefore, the anvil can always be applied with specified impact energy.
In addition, since the number of blows by the hammer and penetration depths according to the number of blows are automatically calculated and accumulated, an N value can be precisely obtained, thereby affording improvements in reliability of test results and convenience in testing.
Therefore, the automatic hammer system for a standard penetration test according to the present invention can contribute to improvements in the accuracy, reliability and convenience of a standard penetration test.